drm/v3d: Switch the type of job-> to reduce casting.

All consumers wanted drm_gem_object * now.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416225856.20264-2-eric@anholt.net
Acked-by: Rob Clark <robdclark@gmail.com>